Various efforts have been made in the past to synthesize polyurethane elastomers showing improved radiation crosslinkability. Thus, EP-A No. 144,065 describes an immersion process for the production of photo- and/or UV-crosslinkable polyurethane safety glass. The polyurethanes are generally based on aliphatic isocyanates. The films thus obtained have to be subjected to a very elaborate immersion process to allow penetration of the unsaturated compounds used for crosslinking based on polyfunctional acrylates or methacrylates with addition of (UV-) radiation-activatable initiators, such as benzophenone derivatives. In spite of this, the crosslinking agents are only present at the surface of the film. This method only gives a very thin layer which can be crosslinked with UV-light. By contrast, it is possible by the presently claimed process to produce shaped polyurethane articles which may have any thicknesses, contain acrylates and/or methacrylates uniformly distributed within the shaped article and undergo rapid and intensive crosslinking by .beta.- and/or .gamma.-rays.
EP-A No. 48,913 describes polyurethanes containing (terminal or lateral) double bonds which are suitable for the production of photopolymerizable recording materials, for example printing plates and photoresist materials, which are subsequently crosslinked by light/UV-radiation. The photopolymerizable polyurethanes are obtained by the reaction of polyether/diisocyanate-based NCO-prepolymers with incorporable bis-acrylate diols. They may additionally contain a low molecular weight acrylate or methacrylate of a dihydric or polyhydric alcohol as polymerizable compounds. They additionally contain photoinitiators and are normally polymerized with UV-light, although electron beams, X-rays and laser beams are also mentioned as actinic radiation. The content of photopolymerizable compounds in the photopolymerizable mixture, which may also contain polymeric binders, is generally from 20 to 100% by weight and preferably from 30 to 60% by weight.
In addition, the prior art describes a number of methods for synthesizing in solution polyurethanes containing ethylenically unsaturated groups through incorporable compounds. These polyurethanes are radically crosslinked (for example with peroxides) to form coatings, preferably in solution in excess ethylenically unsaturated monomers (cf. U.S. Pat. Nos. 4,097,439, 4,073,828, 3,641,199 and 4,153,776).
U.S. Pat. No. 4,436,286 describes polyurethanes which contain double bonds and to which acrylates or methacrylates are added. These polyurethanes are synthesized by an elaborate multistage process. In addition, in contrast to the claimed procedure, these polyurethane elastomers have to contain double bonds in the molecule.
DE-OS No. 3,412,002 describes a process for the production of an extrudable material based on thermoplastic polyurethane elastomers which contains crosslinking agents and which is crosslinkable by electron beams. In this process, the crosslinking agent is added to the polyurethane elastomer granulates in liquid form, and is applied (by rolling) to the surface of standard granulates in such a quantity that, after a predetermined time, the crosslinking agent diffuses completely into the granulate particles of the polyurethane until the surface of those particles appears dry again. The granulate may then be processed by means of an extruder without any danger of clump formation by the granulate particles sticking to one another. The quantity of the crosslinking agent is so large that the radiation dose required for crosslinking need only be high enough to ensure that the polymer chain does not undergo degradation with loss of strength of the polyurethane material in the extruded material. The polyfunctional unsaturated compounds used as crosslkinking agents (in quantities of up to 40% by weight) are, for example, acrylates and methacrylates, allyl compounds, dimaleic imides, epoxidized, polyfunctional or other bifunctional unsaturated compounds.
EP-A No. 80,665 describes a process for the production of crosslinkable sheet-form materials in which a preformed thermoplastic polymer, for example polyurethane or butadiene-acrylonitrile copolymer, is introduced into a twin-screw extruder and melted, mixed in the second zone with low-viscosity ethylenically unsaturated compounds as crosslinking promoters (for example (meth)acrylates of monohydric or polyhydric alcohols), crosslinking aids, such as peroxides, and photoactivators and, optionally, other additives, such as antioxidants, at temperatures of up to 220.degree. C., the resulting mixture is degassed and discharged through flat dies and, finally, is adjusted to the required layer thickness using pairs of heated rolls.
According to EP-A No. 48,913, NCO-prepolymers (based on polyether diols and tolylene diisocyanates) are reacted with certain incorporable bis-acrylate diols in solvents, such as toluene or tetrahydrofuran, optionally in the presence of catalysts or peroxide initiators. The solvents are removed during the shaping process and the shaped articles obtained are optionally crosslinked. However, the polymers may also be crosslinked in solution, the highly crosslinked rubber is comminuted and then molded by compression molding at an elevated temperature.
DE-OS No. 3,444,500 describes non-inflammable, electrical or optical cables of which at least the outer layer of the cable sheath consists completely or partly of radiation-crosslinked polyurethane. Of the various types of polyurethane, a so-called ether type has proved to be the most advantageous because, in addition to its other properties essential to the invention, it is resistant to hydrolysis and immune to bacterial attack.